We just don't know because we just don't have the data or heck, even anecdotal evidence from similar events in the past
However, the utility of the models is to give us a sense of how the different parameters interact. There are parts of the model were we can have a lot of trust, for example that people with severe conditions will need hospitalization, or that people will react quite similar as they reacted yesterday. So for the short term, the models give us quite good guidance, and for the long term, they help to map out scenarios.
So if you actually look at the report in question, you will see that they are actually trying to estimate the impact of various non medical interventions, like encouraging social distancing, by comparing different countries. It is just that newspapers as usual just run with the most immediately digestible number, independent wether that number is important or useful.
The study in question:
https://www.imperial.ac.uk/media/imperial-college/medicine/m...
Some overview video from Dr. Campbell on youtube: (in general, I think his youtube channel is quite good)
If you‘re looking at a simple population infection model, lockdown efficacy is just a factor affecting the contact rate among uninfected actors. You run multiple scenarios for multiple levels of this factor, and see where that gets you.
Or did you mean something else?
(Source: I‘m not an epidemiologist, but as an ecological modeller I work with very similar tools.)
My point is that we can't reasonably predict how people will behave in lockdown past a certain point. We've never had similar lockdowns in a world that was as globalized, as hyper connected as ours. You could go from 2 to 8 weeks of lockdowns if everyone was living in isolated villages a la 1918 Spanish Flu, but that's not our present world.
How do you model a situation where after 4 weeks of lockdowns, a social media post about food shortages goes viral, causing mass panic and breaking of quarantine?
We can't because we've never had a situation like this, or the tools for spreading (mis) information as we currently do.
tl;dr not much
We have old data from 1918.
This is a white swan, not a black one.
More importantly, the mortality, while much higher than flu, it's still relatively low.
Now imagine a virus as contagious as this one, but with 10% mortality over all age groups. That would be unprecedented and probably cause society meltdown.
Anecdote: someone was trying to convice me to panic about Coronavirus because "three hundred and [something] people died just today!"
$ units
8 billion / 80yr
/day
It's nearly 3 million in the US.
Again, our data for all older epidemics is applicable to the epidemic in isolation. But there is no way to accurately model how the epidemic interacts with people simply because the way people live has changed drastically from past epidemics.